Devices that receive signals in one circuit and automatically deliver corresponding signals to one or more other circuits are well known in the art. For example signal repeaters are often used with telephonic circuits, usually amplifying the signal, in order to extend the distance over which a voice signal may be usefully transmitted. Here a signal transmitted though a first circuit is provided to the repeater to be re-transmitted through a second circuit.
These prior art repeaters provide an output signal directly coupled to the input signal being re-transmitted. Therefore, as information appears at the input of such a repeater, so to does this information appear at the output in the form of a re-transmitted signal.
However, this direct coupling of the input and output signals is unacceptable in certain situations. Where the output signal is directly coupled to the input signal, undesired feedback can result. This is so because, by definition, the second circuit is directly coupled to the first circuit in order to provide re-transmission of the input signal. Therefore, where there is also direct coupling of the first circuit to the second circuit a feedback loop, in which the re-transmitted signal may again appear at the first circuit for input into the repeater, is created.
Uncontrolled feedback loops may cause the repeater to become unstable. For example, where the repeater provides amplification of the re-transmitted signal, the feedback loop may, in a very short time, cause the output signal amplitude to cascade, driving the signal beyond the capability of the repeater. Likewise, where the repeater provides a phase or frequency shift in the re-transmitted signal, nulls or otherwise undesirable re-transmitted signals may result from this feedback loop.
It should be appreciated that the aforementioned feedback loops are not a problem in many applications as the first and second circuits are closed and, thus, provide no opportunity for the first circuit being directly coupled to the second circuit. For example, in many telephonic repeater applications the first and second circuits are wireline transmission circuits which are easily decoupled from one another.
However, there are many applications requiring re-transmission of signals in which the aforementioned coupling is present. For example, wireless telecommunications, such as cellular telephony, provides for the transmission of telephonic signals via radio waves. Therefore, re-transmission of such signals is often accomplished via radio wave. However, as the first transmission circuit includes an air-gap and the second transmission circuit also includes an air-gap, direct coupling of these circuits in the form of a feedback loop is common.
In order to avoid undesired feedback, for the reasons discussed above, the input signal must typically be decoupled from the re-transmitted signal. Such decoupling has included such techniques as re-transmission of the signal at a different frequency than the input signal or providing directional re-transmission so as to prevent feedback. Similarly, other techniques, such as reducing the re-transmission power level, have been used to minimize the effects of signal feedback.
However, the prior art techniques of limiting feedback have proved to be less than ideal. For example, in cellular telephony it is often not desirable to provide a re-transmitted signal at a different frequency as such may make both the original and re-transmit frequencies unavailable for use throughout a particular region. Additionally, the use of such different frequencies typically requires additional communication management in order to control the operation of a unit operating through the re-transmitted signal.
Likewise, providing directional re-transmission of the signal is often undesirable as such may cause the re-transmitted signal to insufficiently cover a service area. For example, terrain conditions may cause an irregular shadow in the original signal's radiation pattern. Very often such an irregular pattern is difficult, if not impossible, to properly illuminate with a re-transmitted signal when constrained so as not to provide coupling with the repeater's input.
Similarly, adjustment of the power level of the re-transmitted signal so as to minimize the effects of feedback is often unacceptable. As above, such adjustment often provides inadequate or otherwise undesirable illumination of an area to be covered by the re-transmitted signal.
Therefore, a need exists in the art for a system and method for providing a re-transmitted signal without introducing an unwanted feedback.
A further need in the art exists for a system and method for providing a signal repeater having an output signal decoupled from the input signal.
A still further need exists in the art for providing a decoupled re-transmitted signal requiring a minimum of input signal manipulation so as to provide seemingly transparent signal re-transmission to an intended recipient.